An ongoing dialogue on HIV/AIDS, infectious diseases,
June 21st, 2022
Mayo Clinic Study on Paxlovid Outcomes is Reassuring — but Likely Underestimates Rebound Rate
Over at Clinical Infectious Diseases, researchers from the Mayo Clinic published a retrospective analysis of nirmatrelvir/r (Paxlovid) treatment, with a careful review of each patient’s chart.
The goal was to determine the clinical outcomes after the 5-day treatment course, with a focus on the frequency of rebounds — a topic of great clinical interest but with little real-world data with a decent-sized denominator.
The study included 483 patients with at least some risk factors for severe COVID-19 based on the institution’s Monoclonal Antibody Screening Score. Adjudication of eligibility was done centrally through the hospital’s COVID-19 outpatient treatment program. The mean age was 63, and 93% were vaccinated. Patients were given the option of telemedicine follow-up and encouraged to self-report symptoms or other problems; this information and their full electronic medical records were reviewed after the treatment.
Out of this group, 4 (0.8%) clearly experienced clinical rebound — these cases are described. None of the 4 received re-treatment or were hospitalized. Two other patients (neither of them rebounders) did require hospitalization, but the authors note these were unrelated to COVID-19. There were no deaths.
We can be reassured by the information on the low rate of severe outcomes, an outcome mirrored in low hospitalization rates nationally despite lots of ongoing cases. Rates of hospitalization for each COVID-19 case are now extremely low, especially among those vaccinated, and observational studies (peer-reviewed and published and unpublished) suggest that Paxlovid may reduce this risk even further. A large retrospective study from Kaiser Permanente Southern California involving 5,287 patients found similarly low hospitalization rates.
(Note that these observational studies include patient populations treated with Paxlovid with established risk factors for COVID-19 adverse outcomes — that’s how it’s mostly prescribed under the Emergency Use Authorization (EUA). By contrast, those enrolled in the EPIC-SR study, which was recently stopped due to futility, were “standard risk”. Critically important will be information on baseline characteristics in EPIC-SR.)
For the Mayo study, the authors are to be credited for collecting and reporting the data so quickly, especially at a time when clinicians and patients increasingly observe the rebound phenomenon and wonder what to do about it. The centralized program used for distributing COVID-19 treatments is an additional strength of this report, as it allowed participants the opportunity to self-report progression of symptoms, and upfront ensured that all who were treated met the criteria for high risk based on the EUA.
The big limitation of this Mayo study, however, is that it’s retrospective. As a result, I strongly suspect the 0.8% rate of rebound substantially underestimates the true incidence. Pfizer reported rebounds in 2% of study participants in the prospective EPIC-HR trial, a rate more than twice as high. Based on anecdotal experience — there’s lots of Paxlovid treatment out there! — I would not be surprised if it’s at least 10 times this high, or in the 5-10% range.
What still remains unknown, frustratingly, is whether treatment with subsequent rebound reduces — or paradoxically increases — the total amount of time that people are contagious, or, if the total time is the same, does it just delay the time that a person can confidently say that they’re in the clear?
Why is this important? I’ve had people tell me that they don’t want to take the drug because they’ve heard it might lengthen the time they’re contagious. And I’ve had other patients who experienced rebound express regret on having taken the drug — even though we have no idea whether they would have tested positive for a prolonged period even without the treatment.
Let’s hope further data from prospective studies give us some answers — in particular, the placebo-controlled trials, either Pfizer’s two studies or the large PANORAMIC trial done in the United Kingdom.
In the meantime, I’m still advising people at higher risk for COVID-19 adverse outcomes — even if vaccinated — to be treated with Paxlovid, uncertainties about the above issues notwithstanding.